Enhanced catalytic performance of V₂O₅–WO₃/Fe₂O₃/TiO₂ microspheres for selective catalytic reduction of NO by NH₃

Abstract

The V₂O₅–WO₃/Fe₂O₃/TiO₂ microsphere catalysts were prepared by an impregnation method. The catalytic test results showed that the Fe₂O₃ additives in V₂O₅–WO₃/Fe₂O₃/TiO₂ improved the NO decomposition in the temperature range of 200–400 °C. The characterization results indicated that the iron oxides mainly existed in the form of Fe₂O₃, which was beneficial for the oxidation of NO to NO₂. Meanwhile, the superior catalytic performance of V₂O₅–WO₃/Fe₂O₃/TiO₂ for the selective catalytic reduction of NO has been attributed to its highly dispersed active species, and lots of surface adsorbed oxygen. Prominently, the activity of V₂O₅–WO₃/Fe₂O₃/TiO₂ microspheres can be recovered after cutting off SO₂, and thus these combination metal oxide catalysts show not only high catalytic activities but also good resistance to SO₂.

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1. Introduction

As we know, nitrogen oxides (NO_x) emitted from stationary sources and automobiles are major causes for acid rain, smog, and ozone depletion. The selective catalytic reduction (SCR) of NO_x to N₂ using NH₃ as a reductant is now considered to be the most effective process for the treatment of nitrogen oxides from stationary sources.^1–3 V₂O₅–WO₃ (MoO₃)/TiO₂ has been widely used as an industrial catalyst. This kind of catalyst shows high NO_x reduction efficiencies and the high resistance to SO₂ poisoning. However, the reaction should be operated at relatively high temperatures (350–450 °C) and the SCR reactor is located upstream before the dust removal or desulfurizer device to avoid reheating of the flue gas. Therefore, the catalysts are easily poisoned by the high concentration of dust and SO₂. Thus, the design of the highly active catalysts for low-temperature SCR that is located downstream of the dust removal or desulfurizer device becomes important.

Recently, it has been reported that Fe₂O₃,^4,5 Fe containing mixed oxides^6–9 and Fe-exchanged materials^10–13 exhibit remarkable activity in SCR of NO by NH₃. Liu and He ¹⁴ reported that iron titanate catalysts exhibit high activity at 250–350 °C in the SCR of NO_x with NH₃. This indicated that the iron oxides show better activity at lower temperature than V₂O₅–WO₃/TiO₂. It is commonly accepted that the catalytic behavior of the supported vanadium oxides is sensitive to the intrinsic nature of the vanadium species, including oxidation state, coordination circumstance, dispersion, and stability.^15–18 The chemical and structural features of the promoters affect catalytic performance indirectly, by influencing vanadium dispersion and stabilization of the vanadium oxide species. Herein, iron oxides were incorporated into V₂O₅–WO₃/TiO₂ catalysts to improve the SCR activity at low temperature. Much attention has been given to the synthesis of nano-structured inorganic materials with hierarchical morphologies in the field of catalysis. For example, Yang et al.¹⁹ have reported that the novel WO₃/TiO₂ microspheres show good catalytic performance for the selective oxidation of cyclopentene to glutaraldehyde. The high dispersion of the tungsten species can be achieved on the titania spheres. Here we used TiO₂ microspheres as support. The titania microspheres have a mesoporous structure with a narrow pore size distribution in the nanometer range, and are also thermally stable and have much more surface area. These novel characteristics suggest that the TiO₂ microspheres can be applied as good candidates of catalyst supports. In addition, the promotional effect of iron oxides on the V₂O₅–WO₃/TiO₂ catalysts for the SCR remains unreported.

Therefore, the aim of the present study was to study the promotional effect of iron oxides on the V₂O₅–WO₃/TiO₂ catalysts for the SCR to attain a catalyst with enhanced activity at relatively low temperatures. Fe₂O₃ was firstly employed to coat TiO₂ microspheres (Fe₂O₃/TiO₂ microspheres, denoted FeTMS), and then, we added V, W oxides as additives to get V₂O₅–WO₃/Fe₂O₃/TiO₂ microspheres (denoted VWFeTMS) with improved resistance to SO₂ poisoning for NH₃-SCR. To gain insight into the key role of iron species in the VWFeTMS catalysts in the SCR of NO with NH₃, VWFeTMS catalysts with different iron loadings were prepared by an impregnation method and compared in the SCR of NO with NH₃ at different reaction temperatures in the range of 200–500 °C.

2. Experimental section

2.1 Catalyst preparation

The FeTMS supports were synthesized as follows. The required amount of iron nitrate was dissolved in water. The TiO₂ microsphere materials, prepared following a procedure reported by Guo et al.,²⁰ were added into the stirred solution at 60 °C. Then the excessive water was completely evaporated at 80 °C, and the FeTMS was obtained after calcination at 500 °C in air for 3 h. If the Fe loading was 3 wt%, the support was denoted 3FeTMS. The catalyst was prepared by a conventional incipient wetness impregnation method. The required amount of ammonium vanadate and ammonium tungstate was dissolved in water acidified by oxalic acid. To this solution, the synthesized FeTMS was added and then completely evaporated, and followed by drying and calcination at 550 °C for 6 h under an air atmosphere. Typically, 1V10W3FeTMS represents the loading of V₂O₅ (1 wt%), WO₃ (10 wt%), and Fe (3 wt%), respectively. The catalysts maintained the spherical shape as shown in Fig. S1 (ESI†).

2.2 Characterizations

Nitrogen adsorption–desorption isotherms of the samples were measured at −196 °C using an ASAP 2010Micromeritics, and the corresponding pore size distribution curves are calculated from desorption branches by the BJH method. The specific surface area of the samples was calculated by the Brunauer–Emmett–Teller (BET) method. The SEM micrographs were obtained using a JEOLJSM-6700F scanning electron microscope. The samples were deposited on a sample holder with a piece of adhesive carbon tape and were then sputtered with a thin film of gold. Powder X-ray diffraction (XRD) was performed with a 222 Rigaku D/MAX-RB X-ray diffractometer by using Cu Kα (40 kV, 40 mA) radiation and a secondary beam graphite monochromator. Ammonia temperature programmed desorption (NH₃-TPD), NO_x temperature programmed desorption (NO_x-TPD), and temperature programmed reduction by hydrogen (H₂-TPR) were conducted on TianjinXQ TP-5080 auto-adsorption apparatus, and the NH₃, NO_x and H₂ were monitored by a TCD. Before TPD or TPR, each sample was pre-treated with high-purity (99.999%) N₂ (30 mL min⁻¹) at 300 °C for 2 h. For NH₃-TPD, each sample was saturated with high-purity anhydrous ammonia at 120 °C for 1 h and subsequently flushed at the same temperature for 1 h to remove physisorbed ammonium. The NH₃-TPD analysis was carried out at 120–850 °C at a heating rate of 10 °C min⁻¹. For NO_x-TPD, each sample was saturated with high-purity NO and O₂ at 100 °C for 1 h and subsequently flushed at the same temperature for 1 h to remove physisorbed NO_x. The NO_x-TPD analysis was carried out at 100–850 °C at a heating rate of 10 °C min⁻¹. For H₂-TPR, the flowing gas was switched to 5% H₂/N₂, and the sample was heated to 950 °C at a ramping rate of 10 °C min⁻¹. The laser Raman experiment was performed using an inVia-reflex Renishaw spectrometer equipped with a holographic notch filter, a CCD detector and a laser radiating at 532 nm. The X-ray photoelectron spectroscopy (XPS) was recorded on a Perkin–Elmer PHI 5000C ESCA system equipped with a dual X-ray source, using the MgKα (1253.6 eV) anode and a hemispherical energy analyzer. The background pressure during data acquisition was kept below 10⁻⁶ Pa. All binding energies were calibrated using contaminant carbon (C1s = 284.6 eV) as a reference. The IR spectra were recorded on a Nicolet Nexus 470 Fourier transform infrared spectrometer. The surface characterisation has been performed using pressed disks of pure powders (15 mg for 2 cm diameter), activated by outgassing (10⁻⁵ Torr) at 300 °C into the IR cell. A conventional manipulation/outgassing ramp connected to the IR cell was used. The adsorption procedure involves contact of the activated samples disk with ammonia or NO and O₂ mixed gas at RT at a pressure of 15 Torr and outgassing at 25 °C.

2.3 Catalytic reaction

The SCR activity measurement was carried out in a fixed-bed quartz micro-reactor operating in a steady state flow mode. The reactant gas composition was typically: 550 ppm NO, 550 ppm NH₃, 3% O₂, and balance N₂. The total flow rate was 500 mL min⁻¹ and thus a gas hourly space velocity (GHSV) of 30 000 h⁻¹ was obtained. The temperature was increased from 200 to 500 °C. At each temperature step the data were recorded when the SCR reaction reached steady state after 15 min. The concentration of NO in the inlet and outlet gas was measured by a KM9106 flue gas analyzer. The concentrations of N₂O and NH₃ were measured by a Transmitter IR N₂O analyzer and an IQ350 ammonia analyzer.

3. Results and discussion

3.1 Characteristics of catalysts

3.1.1 Phase composition and surface area. The effect of different iron loadings on the catalysts structure was investigated by XRD techniques. The XRD patterns of VWTMS and VWFeTMS are shown in Fig. 1. The peaks at 25°, 37°, 48° and 53° could be ascribed to the characteristic diffraction peaks of anatase TiO₂ (JCPDS #21-1272). Also there are no peaks observed that are derived from the crystalline V₂O₅, Fe₂O₃, or WO₃, which indicates a high dispersion of vanadia phase as typically found for other oxide-supported vanadia catalysts.^15,21

Fig. 1 X-ray diffraction patterns of various catalysts.

Fig. 2(a) shows the nitrogen adsorption–desorption isotherms of the TMS and other different catalysts. All of them imply the type IV isotherm with an inflection of nitrogen-adsorbed volume at P/P₀ = 0.45 (type H₂ hysteresis loop), indicating the presence of well-developed mesopores in the samples. The pore size distribution measurement was evaluated by the BJH model, as shown in Fig. 2(b); it can be seen that the TMS and different catalysts afford the relatively narrow pore-size distribution with their pore diameters centered at approximately 6.0 nm. The BET model was applied to the desorption branch, resulting in both specific surface areas and pore volumes for these samples. As listed in Table 1, due to the introduction of V₂O₅, nanoparticles within the TMS support are mostly located on the outer surface or presumably infiltrated in mesopores. The BJH desorption average pore diameter is opposite to the surface area. This conclusion can be easily interpreted according to the pores in TMS with different sizes. When the metal oxides are deposited on the surface of TMS or in the small pores, there is the possibility of blocking the Fe₂O₃ and WO₃; the surface areas and pore volumes correspondingly decrease. It is most likely that the embedded entrance of small pores would also block the access to a portion of the smaller pores. Therefore, the more the blocked small-pores, the higher the desorption average pore diameter and the lower the BET surface area.

Fig. 2 (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore size distribution for the samples.

Table 1 Physico-chemical properties of the catalysts

Samples	S BET ^a (m^2 g^−1)	V BJH ^b (cm^3 g^−1)	D BJH ^c (nm)	T d ^d (°C)	Acidic amounts^d (mmol g^−1)
a BET surface area of the samples. b BJH desorption cumulative volume of pores. c BJH desorption average pore diameter (4V/A). d NH3-TPD.
TMS	144	0.266	5.2	—	—
1V10WTMS	80	0.186	7.3	187	0.423
1V10W1FeTMS	87	0.173	6.2	194	0.483
1V10W3FeTMS	88	0.173	6.1	185	0.495
1V10W5FeTMS	60	0.168	8.9	179	0.506

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3.1.2 Surface acidity and redox properties. The TPD of ammonia is a frequently used method for determining the surface acidity of solid catalysts as well as acid strength distribution. The NH₃-TPD profiles of the different catalysts are shown in Fig. S2 (ESI†). The NH₃-TPD results of the different catalysts are summarized in Table 1. Compared with the 1V10WTMS sample, the 1V10W1FeTMS sample has more acidic content and stronger acidic strength, which should be due to the cooperative effects between the iron oxide species and VWTMS that may be helpful to the SCR of NO with NH₃. It is noted that the acidic amounts vary slightly with the change of Fe loadings in the range of 1–5 wt%.

The NO_x-TPD results are shown in Fig. 3. All the catalysts show broad bands between 150 °C and 680 °C. This indicates that there are not only weakly adsorbed NO_x but also nitrate species which exist in the type of monodentate nitrate, bridging nitrate, and bidentate nitrate formed on the catalyst surface.²² The sequence of NO_x desorption amounts from the catalysts is 1V10W3FeTMS > 1V10W5FeTMS > 1V10W1FeTMS > 1V10WTMS. These results mean that the appropriate Fe₂O₃ species can improve basic sites over the 1V10WTMS on which the NO_x species were adsorbed. While excess Fe₂O₃ is not beneficial to the adsorption of NO_x. According to the literature,²³ the active nitrate species could be continuously formed on the fresh FeTiO_x catalyst, which favours the low temperature SCR reaction. Liu et al.²⁴ also reported that more nitrate species on the catalyst surface could participate in the SCR reaction, which was beneficial to promote the SCR reaction. So the addition of Fe₂O₃ species should be positive to improve the low temperature SCR activity of 1V10WTMS.

Fig. 3 The NO_x-TPD profiles of various catalysts.

The TPR is widely used to study the redox property of metal oxide catalysts. Fig. 4 shows the H₂-TPR profiles of VWTMS and VWFeTMS catalysts. As for 1V10WTMS catalysts the main peak is found at 870 °C and the two shoulders appear at 560 and 715 °C. The two peaks at low temperatures are attributed to the reduction of highly dispersed vanadium species, while the presence of a peak at 870 °C can imply the tungsten oxide reduction.^25,26 The addition of Fe to the VWTMS catalysts caused the three reduction peaks to shift to the relative lower temperatures. For the 1V10W3FeTMS catalysts, the three reduction peaks shifted to 475, 680, and 850 °C. Liu and He¹⁴ reported that the Fe³⁺ species have been totally converted into Fe²⁺ until 500 °C, and then partial Fe²⁺ species have been converted into metallic Fe. So the peaks of lower temperatures were attributed to the reduction of V₂O₅ and Fe₂O₃. If overlaying the lines of the H₂-TPR, the peak areas of the VWFeTMS catalysts are far greater than that of VWTMS. Among all the catalysts, the 1V10W3FeTMS catalysts have the largest area. These results indicate that the integration of Fe with the VWTMS catalyst can enhance the oxidative ability of the VWFeTMS catalyst.

Fig. 4 H₂-TPR profiles of various catalysts.

3.1.3 Surface composition and chemisorption properties. The normalized Raman spectra of the catalysts are shown in Fig. 5a and b. The bands at 639, 515, 396, 195 and 143 cm⁻¹ are due to the five Raman active fundamentals of anatase TiO₂.²⁷ In Fig. 5a, due to the stronger peaks of anatase TiO₂, the peaks of V, W and Fe oxides are hard to observe between 200 and 1200 cm⁻¹, so the Raman spectra between 700 and 1200 cm⁻¹ were carefully detected in Fig. 5b. It is noted that the band around 800 cm⁻¹ consists of the overlapping contribution of a second-order anatase feature and of the W–O–W stretching of octahedrally coordinated W units in an environment similar to the one of bulk WO_3,²⁸ and the intensity of this signal increases with the length of the chains. A peak close to 960 cm⁻¹ is reported for the V=O stretch of vanadyl groups internal to metavanadate chains.¹⁶ From Fig. 5b, the bands of 800 and 960 cm⁻¹ are both weaker. Meanwhile, the corresponding XRD patterns did not show any diffraction peaks corresponding to crystalline tungsten and vanadia oxides. This indicates that the dominant tungsten and vanadia oxides over these catalysts are monomeric and polymeric oxide species.

Fig. 5 Raman spectra of various samples (a) full observation between 100 and 1200 cm⁻¹ and (b) deep observation between 700 and 1200 cm⁻¹.

The XPS investigation of binding energies and intensities of the surface elements provides information on the chemical states and relative quantities of the outermost surface compounds. From the V 2p XPS spectra of the VWTMS and VWFeTMS samples given in Fig. 6a, we can distinguish vanadium oxide species in different chemical states from the position of the V 2p_3/2 level, and the peak of V 2p_1/2 (∼524.7 eV) was very weak and hindered by O 1s satellites (521.1 eV). As shown in Fig. 6a, the XPS spectra of the VWTMS and VWFeTMS were too weak for ready analysis indicating that the vanadia is highly dispersed on the TMS support. The V 2p_3/2 of the VWTMS and VWFeTMS is between 514.0 and 517.5 eV which indicates that vanadium species in the VWTMS and VWFeTMS catalyst are in mixed oxidation states of V⁵⁺ and V⁴⁺.²⁹

Fig. 6 XPS spectra of V 2p (a) and Fe 2p (b) and O 1s (c) over different catalysts.

Two peaks ascribed to Fe 2p_3/2 (710.0–711.4 eV) and Fe 2p_1/2 (724.4–725.0 eV) showed up in Fig. 6b. The binding energies of these two peaks correspond well with characteristic Fe³⁺ according to the literatures.^7,30 According to works by Santhosh Kumar et al.,³¹ for SCR, the concentration of accessible Fe³⁺ that can participate in a reversible redox cycle should be maximized, whereas the formation of Fe_xO_y clusters must be completely or largely avoided, indicating that Fe³⁺ may play a major role in the catalytic cycle. The O 1s bands of different samples in Fig. 6c were deconvoluted by the curve-fitting procedure. The sub-bands at lower binding energy (530.2–530.5 eV) corresponded to the lattice oxygen O²⁻ (denoted O_β), and the sub-bands at higher binding energy (531.6–532.1 eV) corresponded to the surface adsorbed oxygen (denoted O_α), such as O₂²⁻ or O⁻ belonging to defect oxide or hydroxyl-like group.³² The quantitative peak-fitting results of O1s can be seen in Table 2. Usually, the surface oxygen O_α is more reactive in oxidation reactions due to its high mobility than lattice oxygen O_β,³³ which is beneficial to the NO oxidation to NO₂ in the SCR reaction. Herein the relative concentration ratios of O_α/O_α + O_β over all the catalysts are calculated and listed in Table 2. The O_α/O_α + O_β ratio of 1V10W3FeTMS catalyst is much higher than that of the other catalysts, indicating the presence of more abundant surface oxygen. This might be beneficial for the enhancement of NO oxidation and thus the SCR activity.^14,34

Table 2 The quantitative results of the mole ratio of different atoms by XPS and peak-fitting results of O1s spectra of different samples

Catalysts	Surface composition^a/mol%			BE (eV)		Oα^b
	W	Fe	Ti	Oα	Oβ
a The average value is provided in the brackets. b Oα(%) = SOα/SOα + SOβ.
1V10WTMS	2.39	—	22.04	531.2	529.5	31.1
	(1.21)		(32.08)
1V10W1FeTMS	2.46	0.30	20.09	531.5	529.6	40.7
	(1.22)	(0.51)	(31.17)
1V10W3FeTMS	6.02	0.89	18.47	530.9	529.6	45.6
	(1.23)	(1.54)	(30.81)
1V10W5FeTMS	2.61	1.52	16.72	531.4	529.5	37.3
	(1.25)	(2.59)	(30.43)

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Table 2 gives the quantitative results of the mole ratios of different atoms by XPS. The XPS spectra of V 2p_3/2 over the VWTMS and VWFeTMS samples were too weak for quantitative analysis. It is interesting to note that the surface W content is much higher than the mean values for all catalysts. It is also clear that the surface Fe content is lower than the average values for all catalysts. These XPS results show that surface enrichment of WO₃ is obviously observed for all of the catalysts, suggesting that WO₃ is dispersed on the surface of the TMS and the Fe₂O₃ is mainly dispersed in the pores of the TMS. According to the comparison of the surface molar ratio of Fe with that of the mean value as added, it can be concluded that the increase of this ratio with the increasing weight percentage of iron is linear. This suggests that the dispersion of the surface iron species is uniform with increasing iron loading.

Fig. 7a shows the in situ IR results of 1V10WTMS and 1V10W3FeTMS catalysts during 500 ppm NH₃ adsorption. Depending on the metal oxide surface, the adsorption of NH₃ can occur through a hydrogen bond, coordination to an electron-deficient metal atom (Lewis acid site), dissociation of NH₃ with formation of NH₂ or NH group, and the formation of NH₄⁺ at Brönsted acid sites.^34–36Fig. 7a indicates that 1V10WTMS and 1V10W3FeTMS have similar adsorption sites. The features centered at 1200 and 1603 cm⁻¹ were attributed to the symmetric and asymmetric deformation vibration of ammonia coordinated to Lewis acid sites. The bands at 1647 and 1450 cm⁻¹ were correlated to NH₄⁺ ions formed on Brönsted acidic sites.³⁴ As compared with the 1V10WTMS catalyst, the Lewis acid sites (1200 cm⁻¹) of 1V10W3FeTMS were weaker, but the Brönsted acid sites (1450 cm⁻¹) were obviously similar. According to the NH₃-SCR mechanism over vanadia–titania catalyst by Topsøe,³⁷ both types of acid sites were involved in NH₃ adsorption. The in situ IR results of NH₃ on 1V10W3FeTMS from 25 to 250 °C were shown in Fig. 7b. With an increase of temperature, the bands of 1200, 1603, and 1647 cm⁻¹ all shifted to the higher wave bands. The intensity of the band at 1450 cm⁻¹ decreased more rapidly than that at 1200 cm⁻¹. This indicated that the Lewis acid sites were much stable than the Brönsted acid sites over the 1V10W3FeTMS catalysts, which indicated that the Brönsted acid sites were more important than the Lewis acid sites at the low temperature NH₃-SCR process. The comparison of NO plus O₂ adsorption on different catalysts was presented in Fig. 7c. According to Liu et al.,²⁴ the bands at 1612 and 1243 cm⁻¹ are attributed to bridging nitrate species, whereas the band at 1581 cm⁻¹ is attributed to bidentate nitrate species. The bands at 1543 and 1296 cm⁻¹ can be assigned to monodentate nitrate. As compared with the 1V10WTMS catalyst, the bands of bridging nitrate species (1612 cm⁻¹) on 1V10W3FeTMS were weaker, but the bands of monodentate nitrate (1296 cm⁻¹) were obviously stronger. The M–O–NO₂(M=Fe) species could rapidly react with adjacent adsorbed NH₄⁺ or NH₃ to produce more reactive intermediates M–O–NO₂[NH4⁺]₂ or M–O–NO₂[NH₃]₂, which could further react with gaseous NO to form N₂ and H₂O.²⁴ The in situ IR results of NO plus O₂ adsorptions on 1V10W3FeTMS from 25 to 250 °C were shown in Fig 7d. With an increase of temperature, the intensity of 1612 and 1243 cm⁻¹ decreased rapidly, while the 1543 cm⁻¹ band still remained. This indicated that monodentate nitrates were more important than the bridging nitrate species for the NH₃-SCR process.

Fig. 7 In situ IR results of (a) comparison of NH₃ on different catalysts at 25 °C; (b) NH₃ on 1V10W3FeTMS from 25 to 250 °C; (C) comparison of NO + O₂ adsorption on different catalysts at 25 °C; (d) NO + O₂ adsorption on 1V10W3FeTMS from 25 to 250 °C.

3.2 Catalytic activity

The effects of various Fe loadings from 1 to 5% on the performance of the 1V10WFeTMS catalyst are shown in Fig. 8. Fig. 8a indicates that NO conversion below 350 °C was sharply improved with the increase of Fe loading. At 250 °C, the NO conversion was generally improved from 52.3 to 81.3% with an increase of Fe loading from 0 to 5%. The catalyst with a 3 wt% Fe loading had the widest temperature window and the best activity. It is worth noting that the 1V10W3FeTMS catalyst showed almost the same activity as the 1V10WTMS catalyst at higher temperature. This indicates that Fe could especially increase the activity of the SCR at low temperature. While the 1V10W1FeTMS catalyst and 1V10W5FeTMS catalyst showed lower activity than 1V10W3FeTMS, it may be attributed to the presence of less surface oxygen on the two catalysts than that on 1V10W3FeTMS. As seen in Fig. 8b, N₂ selectivity over the 1V10WFeTMS catalyst at any temperature is higher than 90%. This indicates that the Fe-doped catalyst displays excellent N₂ selectivity.

Fig. 8 (a) NO conversion against temperature under SCR conditions over different samples, (b) N₂ selectivity against temperature under SCR conditions over different samples. 500 mL min⁻¹ total rate, 550 ppm NO, 500 ppm NH₃, 3% O₂, and N₂ balance, GHSV = 30 000 h⁻¹.

The SCR activities of the different catalysts indicated that Fe, V, and W elements supported on TiO₂ had a positive synergetic interaction and accelerated the SCR reaction (Fig. S3, ESI†). The performance of 1V10W5FeTMS was better than that of 1% V₂O₅–10% WO₃/5% Fe–TiO₂ (common material) (shown in Fig. S4, ESI†). This phenomenon shows that the mesoporous spheres are more favorable for the title reaction. The catalyst exhibited its best performance at a calcination temperature of 550 °C (Fig. S5, ESI†). The NO conversion and selectivity of N₂ over the 1V10W3FeTMS catalyst still kept above 80% and 92% after the second cycle (Fig. S6, ESI†). This finding clearly indicates that the 1V10W3FeTMS catalyst shows good stability which makes possible its further application as a good catalyst.

The effect of SO₂ on NO conversion over the 1V10W3FeTMS, 1V10WTMS and 3FeTMS catalysts was shown in Fig. 9a. When 100 ppm SO₂ was added to the reaction gases, the NO conversion over the 1V10W3FeTMS decreased from 97% to 75% in 1 h, and then nearly stabilized. After turning off SO₂, the activity restored to 95%. The NO conversion over the 1V10WTMS decreased from 80% to 61% in 1 h after SO₂ was added to the reaction, and then nearly stabilized. After removing SO₂, the activity recovered to 71.8%. While the NO conversion over the 3FeTMS decreased from 94% to 70% in 1 h, and then nearly kept. When shutting off SO₂, the activity restored to 76.6%. This indicates that the combination effect of iron oxide and V₂O₅–WO₃/TiO₂ could enhance the 1V10W3FeTMS catalyst resistance to SO₂.

Fig. 9 (a) The effect of SO₂ on NO conversion over the 1V10W3FeTMS, 1V10WTMS, and 3FeTMS catalysts. (b) The effect of SO₂ and H₂O on NO conversion over the 1V10W3FeTMS, 1V10WTMS catalysts. 500 mL min⁻¹ total rate, 550 ppm NO, 500 ppm NH₃, 3% O₂, 10% H₂O (when used), 100 ppm SO₂ (when used) and N₂ balance, and 3FeTMS 320 °C, GHSV = 30 000 h⁻¹.

The effect of SO₂ and H₂O on NO conversion over the 1V10W3FeTMS, 1V10WTMSand 3FeTMS catalysts was shown in Fig. 9b. When 100 ppm SO₂ and 10% H₂O were added to the reactants, the NO conversion over the 1V10W3FeTMS catalyst rapidly decreased from 97% to 64% in 1 h, and then nearly stabilized. After turning off SO₂ and H₂O, the activity restored to 75%. The NO conversion over the 1V10WTMS catalyst decreases from 80% to 53% in 1 h, and then nearly kept. After removing SO₂ and H₂O, the activity restored to 61%. For the 3FeTMS catalyst, the NO conversion reduces from 94% to 62% in 1 h, and then nearly stabilizes. After shutting off SO₂ and H₂O, the activity recovered to 67%. These results indicate that the resistance to the mixture of H₂O and SO₂ over the 1V10W3FeTMS catalyst is stronger than the 3FeTMS catalyst. This indicates that the addition of V₂O₅ and WO₃ oxide could increase 3FeTMS catalyst resistance to SO₂ and H₂O. The decreased activity is mainly caused by the tightly adsorbed SO₂, the formed sulfates, and the other species on the surface occupy the active sites, which decrease the SCR activity.

For the SCR of NO_x with NH₃, two reaction routes have been proposed: the “standard” reaction occurring between NH₃ with NO, and the “fast” SCR reaction where both NO and NO₂ are involved. The “fast” path has an important contribution, particularly at low temperature. So the generation of NO₂ from NO is a key factor.³⁸ The conversion of NO to NO₂ over the different catalysts was measured and listed in Fig. S7 (ESI†). It is obvious that the introduction of Fe additive to the catalyst increases the oxidation activity of NO to NO₂. It has been accepted that NO is more readily reduced to N₂ along with a portion of NO₂ than single NO by NH_3.^39,40 The introduction of iron oxide increases the oxidation rate of NO to NO₂, which increases the SCR activity.

3.3 Correlation between catalytic activity and structures

It is confirmed that 1V10W3FeTMS exhibits the best activity among all the employed catalysts in the SCR reaction. It is demonstrated that the active components of vanadium oxide and tungsten oxide were well-dispersed on the surface of the iron oxide coated TiO₂ microspheres, and the integration of Fe³⁺ with the VWTMS catalyst can enhance the oxidative ability of the VWFeTMS catalyst. It is clear that the dominant tungsten and vanadia oxides over these catalysts are monomeric and low polymeric oxide species. Based on the characterization results above, the surface oxygen O_α is more reactive in oxidation reactions due to its high mobility than lattice oxygen O_β, which is beneficial to the NO oxidation to NO₂ in the SCR reaction. Herein, the O_α/O_α + O_β ratio of the 1V10W3FeTMS catalyst is much higher than that of the other catalysts, indicating the presence of more abundant surface oxygen. We proposed electron transfer between V and Fe in Scheme 1. The V and Fe species are interconnected in the form of V–O–Fe through oxygen bridges, facilitating electron transfer. This is beneficial for the enhancement of NO oxidation and thus the SCR activity. This agrees with the experiment of the conversion of NO to NO₂ over 1V10W3FeTMS catalysts. The NO_x-TPD results show that the appropriate Fe₂O₃ species can improve basic sites over the 1V10WTMS on which the NO_x species were adsorbed and the active nitrate species could be easily formed resulting in the enhanced low-temperature SCR activity of the 1V10WTMS.²³ In addition, as compared with the 1V10WTMS catalyst, the monodentate nitrate of the 1V10W3FeTMS was obviously more. The M–O–NO₂ (M=Fe) species could rapidly react with adjacent adsorbed NH₄⁺ or NH₃ to produce more reactive intermediates M–O–NO₂[NH₄⁺]₂ or M–O–NO₂[NH₃]₂, which could further react with gaseous NO to form N₂ and H₂O.⁴¹

Scheme 1 A redox catalytic cycle of the low temperature SCR reaction over VWFeTMS catalysts.

4. Conclusion

The catalytic activity of 1V10WTMS was greatly enhanced by the addition of 3 wt% of Fe in the temperature range of 200–500 °C. V₂O₅ and WO₃ could enhance the 3FeTMS catalyst resistance to SO₂. The XRD results indicate that the active components of V and W oxides are well-dispersed. The Raman spectra indicate that the dominant vanadia and tungsten oxides over these catalysts are the monomeric and oligomeric tetrahedrally coordinated vanadia and tungsten species. NO_x-TPD results show that the appropriate Fe₂O₃ species can improve basic sites over the 1V10WTMS on which the NO_x species were adsorbed. The H₂-TPR indicated that the addition of Fe over the VWTMS catalyst enhanced the oxidative ability of the VWFeTMS catalyst. The O_α/O_α + O_β ratio of the 1V10W3FeTMS catalyst is much higher than that of the other catalysts resulting in the enhancement of NO oxidation and thus the SCR activity. Moreover the IR-NO + O₂ results show the monodentate nitrate species are essential for the SCR reaction.

Acknowledgements

The authors acknowledge the support of National Natural Science Foundation of China (51108258), Science and Technology Commission of Shanghai Municipality (10540500100 & 11nm0502200), Research Fund for the innovation Program of Shanghai University (A.10040711003), Shanghai Municipal Education Commission (B.37040711001), and Key Subject of Shanghai Municipal Education Commission (J50102). The authors would like to thank Prof. Y. L. Chu and Prof. W. J. Yu from analysis and test center of Shanghai University for help with the SEM and TEM measurements.

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Footnote

† Electronic supplementary information (ESI) available: The morphology of 1V10W3FeTMS catalysts, and the catalytic performance of samples with various fabrication parameters. See DOI: 10.1039/c2cy20332d

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